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11CostSavingsforMoldMaterialsConsidertheDetailsWaystoreducecoststhatarenotrelatedtothepriceperpoundofthemoldmaterial.ByPatriciaMillerChoosingtherighttoolingmaterialforplasticmoldingisbecomingevermoredifficult.Inlightofintensecompetition,aswellasthecostofrawmaterials,whichisdrivingthepriceofmaterialsformoldsup,itbecomesincreasinglymoreimportanttobeselectiveinthechoiceofmoldmaterials.Therearethingsthatcanbedonetohelpthemoldmakermakethebestselectionsfortheapplicationathand,andthesethingsarenotsimplylookingatthepriceperpound.Butinordertodothis,themoldmakermustconsiderotherfactors.Thisstartsatthebeginningofthemoldmakingcycle,astheparttobemanufacturedisbeingproposed.ThemajorareastobeconsideredinamolddesignincludeSizeandcomplexityofthepartormoldProductionquantityrequiredTypeofplasticmoldingmaterialrequiredanditsimpactonthemoldingenvironmentMechanicalrequirementsforthemoldPhysicalpropertyrequirementsforthemoldthermalconductivity,stiffness,thermalexpansionStabilityrequirementofmoldduringoperationAssemblyissuesmatingmaterialcriteria,coatabilityDesignfeaturessharpcorners,thinsections,sealingmethodsSurfaceconditionrequirementspolishing,texturingdemandsManufacturingmethodselectrodischargemachining,hardmillingCostsavingscanoccurinalltheseareas,butforthisdiscussionfourareasofalloyswillbeaddressed1.Newhighhardnessmatrixalloys2.Thermallyconductivealloys3.Corrosionresistantmaterials4.PrehardenedalloysIneachcase,thecostofthemoldmaterialissignificantlyoutweighedbythebenefitsthesealloysbring.22Figure1.Amoldforplasticinjectionmoldedelectricmotorrotors,madeof30percentglassfilledPolyamide.Moldmaterialhas,todate,increasedthelifeofthemoldover20percent,eliminatingmoldrepairsandrefurbishment.FigurescourtesyofBohlerUddeholmCorporation.1.HighHardnessMatrixAlloysInanexcitingdevelopmentformoldmaterials,therearenowtoolsteelgradesavailablethatcanreplaceS7,H13andwithcoatingreplaceA2,D2andM2typeswherewearresistanceisrequired.Buttheadvantagethatthesegradesbringisthattheycanbeusedfrom50HRCupto62HRC,andareweldable,polishabletohighlevels,texturableandarecoatablewhenevenhigherwearresistanceisrequired.Figure2.Therelationshipofhighlyconductivealloys,thermalconductivityversushardness.ThenewCuNialloy,havinghardnesslevelslikethatofP20,hashigherthermalconductivitythanothercopperalloysandaluminum.Verytoughgrades,thesematerialsalsoprovideanaddedadvantagethatthermalconductivityexceedsthatofH13,hencecycletimescanbereduced.Thesegradesreplacepastcoldworkgradesthatcouldonlyachievetheirmechanicalpropertiesbylowtemperaturetempers,whichdidnotpermitgoodweldability,nitridingorPVDcoatingabove400oF.Theirhighpolishabilityandtexturabilityisduetotheexcellentuniformityofthematrixfromtheuseofhightechnologyremeltingprocesses.AnexampleofthisisshowninFigure1.AninjectionmoldinsertmadeofPolyamideplus30percentglassfiber,wasmanufacturedfromachromiummolybdenumvanadiumalloyedtoolsteel.At5456HRC,ithasbeenrunningforover7,700piecesandisstillrunningwell.PremiumH13at5052HRCbegantowearandplasticallydeformat6,000pieces.2.ThermallyConductiveAlloysCopperalloyshavebeenavailableforseveralyearstoaddresstheneedforcycletimereductionandpartreproducibility,andhavebeenusedparticularlyforcoreswhereplasticresidingtimesarehighest.Copperberylliumalloysareavailableinhardnessrangesof3040HRC.Inaddition,thereisacoppernickelalloythatcanachievehardnessof30HRC,whichisintherangeofatypicalP20.Theadvantagethisgradebringsisthatitsthermalconductivityexceedsthatofothercopperalloysinthishardnessrange,andalsothatof33aluminum.ThisgradealsoisantigallingandcorrosionresistantseeFigure2.Figure3.RoughmillingofthecavitypremiumH13,45HRC.3.DesignandStainlessAlloysItistimetoreconsiderstainlessalloys.Therearenewstainlessalloyswhosebenefitsareclearhighpolishability,withtoughnesslevelsintherangeofPremiumH13,upto5052HRC.Thefactthatthesealloyswillmaintaintheircorrosionresistancewithminimalneedforreworkorrepolishingoverthelifeofthemold,andstillprovideadurable,highmechanicalstrengthmold,isworthcarefulconsideration.Figure4.DrillingofcoolingchannelspremiumH13,45HRCOneareawhichhaslimitedthelifeofstainlessmoldsinthepasthasbeentheuseoftaperedpipeplugs.Heattreatmentlimitations,machiningissueswithdevelopingthethreads,stressesgeneratedinthethreadsfollowingtorquing,alongwiththecorrosiveconditionsofdeadzoneswhichcreatepittingattackonthestainlesscanleadtocrackingintheseregions.Newstainlessalloyscanminimizesusceptibilitytothisalongwithplugdesignsthatareavailabletohandlehydraulicsealingissueswithoutmachiningthreadsintothemoldmaterial.4.PrehardenedMoldMaterialsLookingawayfromthesteelcosttomanufacturingtechnique,